The bony pelvis, which, covered by soft tissues, forms the lower support of the abdominal contents, is formed by two ossa innominata fixed behind to the sides of the sacrum and meeting in front at the symphysis. It forms a firm and strong base on which the trunk can rest, and through which its weight can be transferred to the free limbs below.

The sacrum and coccyx have been described already (p. 38), but it is necessary to give a short description of the various parts of the innominate bone before considering the pelvis as a whole.